<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>float</title>
  <style>
    *.clear {
      clear:both;
      list-style:none;
    }
    #nav {
      border:1px solid red;
    }
    li.nav_li{
      list-style: none;
      float:left;
      border:1px solid #aaa;
      padding:10px 25px;
    }
    #content {
      border:1px solid black;
      /*设置了float之后，如果增加了边框，子容器宽度就会发生改变，如果超过了父容器的宽度，就会换行*/
      width: 996px;
    }
    #left,#center,#right {
      width:300px;
      height:300px;
      float:left;
      padding:10px;
    }
    #left {
      border:1px solid red;
    }
    #center {
      border:1px solid green;
    }
    #right {
      border:1px solid blue;
    }
  </style>
</head>
<body>
  <ul id="nav">
    <li class="nav_li">导航信息1</li>
    <li class="nav_li">导航信息2</li>
    <li class="nav_li">导航信息3</li>
    <li class="nav_li">导航信息4</li>
    <li class="nav_li">导航信息5</li>
    <li class="clear"></li>
  </ul>
  <div>具体的内容</div>
  <div id="content">
    <div id="left"></div>
    <div id="center"></div>
    <div id="right"></div>
    <div class="clear"></div>
  </div>
</body>
</html>